I haven't modded morrowind for ages so I can't pinpoint all, but the things I found most changeable were these:
Most movement (scripting) has turned into animations.
You can Parent/Child link references now for easier scripts.
Quests are the source of most basic things now.
AI has taken a much more important role as no npc is standing still without a reason.
It's a much longer list of course...Personally it took me a while to get used to the transition but it's got easier and easier along the way.
(You can say pretty much the same about the new gamesas forums
.)
As said, most things can be picked up easily and a really recommended thing is to follow the beginners tutorial as anything you already know should just allow to speed things up through those while you get to know more bits about oblivion modding as well.
Other than that, it's just the basic trial and error as ever
.